Studies On Oxide Cathode Materials For Li-/Na-ion Batteries With High Energy Density

Li-ion and Na-ion batteries(LIBs and NIBs)are becoming more and more attractive as important energy storage systems in recent years.LIBs are widely commercialized on portable electronic devices and electric vehicles at present,and the demands still keep increasing.As for NIBs,the potential of being utilized in large scale energy storage devices is worth expecting with its cost advantage.Cathode materials are the decisive factor of the energy density for either LIBs or NIBs.For LIBs,the energy density of commercialized cathode materials is approaching the theoretical limit.Therefore,cathode materials for the next generation with higher energy density,Li-rich materials as an example,are being needed.On the other hand,there is not a material that can dominate in cathodes for NIBs,thus the comprehension and research on NIB cathode materials need to be deepened.As for electrode materials,it is structure and chemical components that determine the electrochemiical properties.In this article,Li-rich materials Li2Ru1-xNixO3-? and Li1.2Mn0.4Ti0.4O2,and 03-type NaLi0.1Ni0.35Mn0.55O2 for NIBs are studied for learning the relationship between the structure and electrochemical properties,and the function of different elements in the materials.Oxygen redox is significant for Li-rich materials,as it provides a large amount of capacity.However,the reversibility of oxygen redox still needs to be improved,thus a deeper understanding of this reaction is needed to further utilize the capacity from oxygen redox.On the other hand for 03-type Ni/Mn based NIB cathodes,a direct 03-P3 phase transformation can do harm to the eye]ability.Therefore,a modification on the bulk structure is needed to improve the phase transformation process.A series of Ni-substituted Ru-based Li-rich materials Li2Ru1-xNixO3-?(LRNxO,x=0,0.1,0.2,0.3,0.4,0.5)were synthesized by a solid-state method.Both X-ray diffraction(XRD)and Neutron(ND)patterns shows a phase transformation from layered structure to an ordered rocksalt(OR)structure as Ni amount increases.First,LRO exhibits a layered structure.After a Ni substitution,the materials shows a coexistence of layered and OR structure for LRN0.1O and LRN0.2O.When x in LRN,O>0.3,a pure OR structure is observed.Electrochemical test results display that the O oxidation potential is lowered as Ni amount increases,from 4.27 V to-4 V,which corresponds to the results of X-ray photoelectron spectroscopy(XPS)and X-ray absorption spectroscopy(XAS).O2n-is observed at 3.85 V charge state from O ls XPS spectra of LRN0.4O,implying a lowered O oxidation potential.In addition,the growth of the pre-edge peak in the XANES spectra at the Ru K-edge of LRN0.4O from-3.8 V upon charging can also be an indirect evidence of oxygen oxidation.First Principles Calculations were also carried out to confirm and explain the phenomenon.There is only one kind of O chemical environment in layered LRO,while Ni substitution itself,and the phase transformation caused by Ni substitution diversify the O chemical environment,and thus change the density of state(DOS)of O.The overall DOS of LRNO is closer to the Fermi level compared with that of LRO,which confirms that the O oxidation potential is indeed lowered.Besides,in situ XRD data shows the good stability of the frameork during charge and discharge processes,and Li+can insert into tetrahedral sites upon overlithiation according to ss-NMR spectra.A cation-disordered rocksalt Li-rich material Li1.2Mn0.4Ti0.4O2(LMTO)was synthesized by a solid-state method.Both XRD and ND patterns are indexed to the cation-disordered rocksalt structure.After ball-milled without carbon(LMTO-B)and with carbon(LMTO-C)the particle size decreases,and only the electrochemical properties of LMTO-C is obviously improved.The discharge specific capacity of the first cycle of LMTO-C increases from 108 mAh g-1 to 249 mAh g-1.However,the problem of voltage and capacity decay needs to be solved.The mechanism of voltage and capacity decay were preliminarily studied by in situ XRD,XAS and well-designed electrochemical tests.In situ XRD results show a 10%volume change upon charging and discharging,which is speculated to be one of the main reasons for capacity/voltage fade.XANES data demonstrates that Mn oxidation is observed below 4.4 V and O oxidation above 4.4 V during first charge process.Therefore.the 0 redox can be excluded when the voltage range is limited to 1.5-4.3 V.The capacity loss is mitigated in such case,while the voltage fade is still obvious.This indicates the poor reversibility of oxygen redox is another reason for capacity loss,and the voltage fade still exists even though there is only transition metal(TM)redox during charge and discharge process.03-type layered oxide NaNi0.5Mn0.5O2(NNMO)and Li-substituted NaLi0.1Ni0.35Mn0.55O2(NLNMO)were successfully synthesized by a co-precipitation method.XRD patterns display that a small amount of O'3 phase appears in NLNMO after Li substitution.The cyclability of NLNMO is much better than that of NNMO,and the capacity retention after 100 cycles increases from 50%in NNMO to 85%in NLNMO.In situ XRD results show that the O'3 intermediate phase will appear during cycling for NLNMO,while for NNMO there is an absence of O'3,which is the main reason of the cyclability difference between the two materials.Solid state NMR data explains why the introduction of Li+can contribute to the formation of the intermediate O'3 phase.Li+was observed in both TM layers and Na layers,leading to vacancies in TM layers.This is speculated to be the reason for the formation of O'3 after Li substitution.